From: Per Axelsson
I have a 1987 Plus 8 and I have decided to change to a twin exhaust system. Is there a difference in the performance between cast and tubular manifolds?
Dear Per,
The answer is "no two things are the same even if they appear identical". That being said, a cast manifold holds heat longer than the tube type. Also cast is heavier and in a competition situation lighter is better and the car is faster as a result. But these are still minor points. FLOW is what you are looking for. The idea today with an exhaust system is to get rid of the exhaust as quick as possible. In the past, there was a tendancy towards intricate exhaust systems that twisted and turned in an attempt to tune the performance of the engine and dump the exhaust out. Those systems looked good but have have been found to not be as effective as good short tubes that get the exhaust out of the engine quickly, simply and as fast as possible.
This type of simple effective system is not always easy
for a street application where you are required to
to hang mufflers, cat converters and smog pumps on the
end of your tubes so that the engine is forced to further "re-digest" what
it is trying to get rid in the first place.
In sum, if you are into COMPEITION you want tube exhaust (headers) with about a foot to foot and a half straight out or turned down tubes with only a noise restrictor at the end of the collected pipes (like a Super Trap) and that is it.
For the STREET, tubes would also be the first choice with a small free flow muffler (to cut down noise from a legal stand point) to get the best results. On the other hand a cast system such as the SD1 units used on the Plus-8s work very well also (heavier, hold heat longer but still flow well) again with a small free flow muffler installed. However, even with this information your final decision may turn out to be a function of your local laws of your city, state, or country. On my 1983 Street Plus-8 I use the SD1 exhaust system with a straight pipe attached to the header tubes which runs all the way to the rear of the car. The pipe passes behind the rear wheel between the chassis rail and backside of the tire and is topped off at the end of the pipe with a Ansa exhaust tip (twin pipes and tips). I end up with good flow, performance and a mellow exhaust note. For the racing Plus-8 it is tube exhaust with one and one half feet turned slightly down and back towards the cars rear with four pipes joining one collector pipe with a supertrap on the end and that's it for max performance.
Best to you.JHSII
There is a substantial difference in performance (AND SOUND !) with a dual exhaust system provided you use the tubular 4 branch manifolds/twin branch downpipes and then "straight through" silencers (just one each side)...probably up to 10% increase in power. (This system is in fact an amalgam of the manifold extractor system used on +8 between 1977 and 1985 together with the silencers and tailpipes used from 1971 to 1976.).
Do remember that the fitting of a dual system where a
single system was the original fitting, involves a lot of work, as the
valances (inner wings) have to be recut and flanged to allow the manifolds
to exit correctly...also on later cars with a brake servo (1993 on) the
fluid tank from the master cylinder sometimes needs to be moved up to the
bulkhead area (in effect making it a remote tank) in order to keep it clear
of the heat from the manifolds...this is not a problem with non-servo cars
and only rarely with LHD cars anyway. In either case it is worth the trouble
in order to regain the traditional V8 sound and the extra performance!
John Worrall